In a transmission system, such as a wireless or wireline communication network, echo cancellers are typically utilized to provide echo cancellation. As a signal (referred to herein as a “reference signal”) is being transmitted from a remote location through a full duplex transmission system, an echo of this reference signal will generally appear, at the remote location, coupled with a local (near end) signal being transmitted from a local or near location to the remote or far location. Generally, such an echo may be either or both an electrical echo, such as from impedance mismatches (e.g., from a 2-4 wire hybrid), or an acoustic echo, from an acoustic path such as a speaker-microphone coupling of a speaker phone or a hands-free mobile telephone. This signal having both the echo signal (a delayed and typically reduced power version of the reference signal) and the local, near end signal, is referred to herein as the “combined signal”. For example, without echo cancellation, a person at the remote location would hear the speaker's voice from the local location, plus a delayed version (echo) of what the person at the remote location has said previously. In addition to voice transmission, such echoes appear in data transmission, multimedia transmission, and other forms of information transmission, irrespective of transmission media such as air, wireline, cable, fiber, and so on.
Echo cancellers, which are used to remove the echo portion of the combined signal to form an echo cancelled signal for transmission to the remote location, are generally designed to cancel echoes having variable delays up to a maximum specified delay, such as a maximum delay of 64 ms. Typical echo cancellers, implemented as adaptive filters, generate a synthetic echo based upon training of the filter in the transmission system. The synthetic echo is preferably a close match to the actual echo, and the synthetic echo is subtracted from the combined signal, by the echo canceller, to produce an echo cancelled signal. Under various circumstances, however, depending upon network conditions, such as heavy traffic conditions, there may be an increased delay beyond the capacity of the echo canceller employed. When the delay of the incident echo exceeds this maximum amount, the echo canceller treats the echo as a local or near end signal, such as near end speech, and allows it to be transmitted to the remote location. As a consequence, the echo canceller is unable to cancel the echo, resulting in an unacceptable communication session.
Various prior art methods of determining echo delay, and inserting a bulk delay to accommodate an echo, have significant deficiencies. Many are open-loop systems, and are unable to provide actual measurements of echo return loss and, as a consequence, cannot coordinate with and release control from an echo canceller to allow for delay insertion and echo suppression. Such open-loop structures are also subject to significant interference. Other prior art techniques, including those using cross-correlation methods, are unable to prevent echo leakage during retraining of an echo canceller and are unable to provide echo suppression.
A need remains, therefore, for an apparatus and method to provide echo cancellation under circumstances in which the delay in the echo path exceeds the delay capacity of the echo canceller. Such an apparatus and method should respond to conditions of greater delay in the echo path, and enable an echo canceller to remove the echo from the combined signal. In addition, such an apparatus and method should be robust under conditions of interference, should coordinate with an independent echo canceller, and should provide echo suppression.